Unexpected results from an XML query filter for security event log

Try this:

<QueryList>
  <Query Id="0" Path="Security">
    <Select Path="Security">*[System[(EventID=4608)]]</Select>
    <Suppress Path="Security">*[EventData[Data[@Name="TargetUserSid"] = "S-1-5-18"]]</Suppress>
  </Query>
</QueryList>